Plant Physiology - Musiyenko M.M. 2001

Respiration
The pentose phosphate pathway of oxidation and its role in cellular metabolism

Glycolysis is well known as the primary pathway of hexose Catabolism. However, plants also utilize an alternative pathway—the Pentose Phosphate Pathway, or hexose monophosphate shunt.

The term "shunt" indicates that this pathway branches off from the main glycolytic route. The Enzymes of this shunt, much like those of glycolysis, are localized in the Cytosol, as well as in proplastids and METABOLISM/14.html">Chloroplasts.

First of all, it should be noted that while glucose oxidation via glycolysis, the Krebs cycle, and the Electron Transport Chain primarily channels its energy into ATP synthesis, The Pentose Phosphate pathway generates a different type of metabolic energy—reducing power in the form of NADPH₂ molecules. This occurs because a fraction of the electrons and protons from the respiratory substrate is diverted into biosynthetic processes rather than being transferred to O₂.

The most readily available source of reducing equivalents within The Cell is NADPH₂, which differs from NADH₂ by the presence of a phosphate group at the second carbon atom (C₂) of one of its ribose components. The essential difference between them lies in their distinct physiological roles: NADH₂ is oxidized in The electron transport chain to generate ATP, whereas NADPH₂ serves as a hydrogen and electron donor for various reductive biosyntheses in The plant cell.

The Discovery of the first enzyme of this cycle (glucose-6-phosphate dehydrogenase) by Otto Warburg in 1931 paved the way for its complete elucidation, which was later accomplished by F. Dickens, F. Lipmann, B. Horecker, and E. Racker. This respiratory pathway operates exclusively under aerobic conditions and serves as the primary cellular source of pentose sugars, which are essential components of ATP, NAD, FAD, and Nucleic Acids. It can function alongside glycolysis, supplying anywhere from 10% to 99% of the energy yielded by carbohydrate breakdown during Respiration.

Class="center">The pentose phosphate shunt branches off from The Glycolytic Pathway at the level of glucose-6-phosphate (Fig. 104) and ultimately re-enters the glycolytic sequence at the level of fructose-6-phosphate and glyceraldehyde 3-phosphate:

Fig. 104. Sequence of reactions in the pentose phosphate pathway

Thus, the pentose phosphate hexose oxidation shunt performs several core Functions: first, it supplies NADPH, which acts as a reductant in various biosynthetic processes when photosynthetic production is absent. Therefore, it is of particular importance for non-photosynthetic Tissues (during tissue differentiation and seed germination), as well as during dark periods;

second, the cycle produces ribose-5-phosphate, which is essential for the Biosynthesis of Nucleic acids and NUCLEOTIDES, CoA, and the Coenzymes NAD⁺, NADP⁺, and FAD⁺.

Finally, it yields erythrose-4-phosphate, which is used for the synthesis of shikimic acid—a precursor to aromatic rings, Vitamins, Lignin, and Tannins. It also serves as a source for CARBOHYDRATES with diverse carbon skeletons.

The pentose phosphate pathway is favored when the cell has a high demand for pentoses and requires NADPH as an energy source. Products of this oxidative pathway, such as ribulose-1,5-bisphosphate and NADPH, can also participate in dark fixation and the reduction of CO₂ during Photosynthesis.

In essence, the pentose phosphate shunt closely mirrors The Calvin Cycle (with only two reactions being specific to photosynthesis and the rest identical).

In young, particularly meristematic tissues, glycolysis predominates, whereas older tissues rely heavily on the pentose phosphate pathway as well. This pathway also operates during various Types of Fermentation. For instance, the intermediate xylulose-5-phosphate can be cleaved by phosphoketolase during Lactic acid fermentation to yield acetyl phosphate and glyceraldehyde 3-phosphate (GAP).

The pentose phosphate shunt can also be incomplete, as it may transition into the glycolytic breakdown of carbohydrates at a certain stage. The oxidative pentose phosphate pathway is divided into two phases. The first phase involves The conversion of glucose-6-phosphate into ribulose-5-phosphate with the generation of 2 molecules of NADPH. Phase I begins with The oxidation of glucose-6-phosphate to 6-phosphoglucono-δ-lactone. This reaction is catalyzed by NADP-dependent glucose-6-phosphate dehydrogenase. Subsequently, 6-phosphoglucono-δ-lactone is hydrolyzed by 6-phosphogluconolactonase to form 6-phosphogluconic acid. The latter undergoes oxidative decarboxylation to yield ribulose-5-phosphate, a reaction catalyzed by NADP-dependent 6-phosphogluconate dehydrogenase. Thus, in the first phase, two molecules of NADPH are produced per molecule of glucose-6-phosphate. In the second phase, ribulose-5-phosphate molecules undergo a series of transformations, resulting in the resynthesis of glucose-6-phosphate and glyceraldehyde 3-phosphate. This occurs as follows. In Phase 2, ribulose-5-phosphate is isomerized into ribose-5-phosphate and xylulose-5-phosphate through the action of ribosephosphate isomerase and ribulose-5-phosphate 3-epimerase, respectively. Ribose-5-phosphate and xylulose-5-phosphate then interact (in a transketolase-catalyzed reaction) to yield glyceraldehyde 3-phosphate and sedoheptulose-7-phosphate. These compounds, in turn, react via transaldolase to form fructose-6-phosphate and erythrose-4-phosphate. In the presence of transketolase, erythrose-4-phosphate and xylulose-5-phosphate react to produce glyceraldehyde 3-phosphate and fructose-6-phosphate. The latter can then enter the glycolytic pathway.

The pentose phosphate shunt and glycolysis are interconnected by the enzymes transaldolase and transketolase, which catalyze Reactions Involving the rearrangement of carbon skeletons:

Through the second phase, any excess pentoses can be redirected into the glycolytic breakdown pathway (Fig. 105). Furthermore, when necessary, ribose-5-phosphate and erythrose-4-phosphate can be synthesized from glyceraldehyde 3-phosphate and fructose-6-phosphate without the need to generate NADPH.

Fig. 105. Oxidative pentose phosphate pathway of carbon Skeleton rearrangement

As seen from the reactions of glucose oxidation in the pentose phosphate cycle, they are quite similar to the carbohydrate conversion reactions in the photosynthetic Calvin cycle. Due to the presence of identical intermediates, these processes are likely interconnected at certain points. Moreover, it is known that alongside its localization in the Cytoplasm, the pentose phosphate pathway also operates in chloroplasts in the dark. Consequently, during the ontogeny of a plant Organism and depending on specific environmental conditions, shifts occur between the potential pathways of Carbohydrate Catabolism.

The rate of this pathway is regulated by the concentration of NADP⁺, whereas the proportion of glucose-6-phosphate channeled into it depends on the cellular demand for NADPH, ribose-5-phosphate, and ATP. A molecule of glucose-6-phosphate can be completely oxidized to CO₂ with the simultaneous generation of NADPH, yielding 2 molecules of NADPH₂ per carbon atom oxidized.

Direct Sugar Oxidation

The direct glucose oxidation pathway has been discovered in certain Bacteria, Fungi, and animals, as well as in marine Algae (data for higher plants remain inconclusive). The oxidation of glucose to gluconic acid is mediated by the enzyme glucose oxidase. Prior to oxidation, mutarotation of α-glucose into the β-form takes place, catalyzed by the enzyme mutarotase. Through a series of transformations, gluconic acid ultimately yields two trioses: pyruvic acid and GAP, which can subsequently be oxidized in the Krebs cycle via Pyruvate.

If other sugars undergo direct oxidation during respiration, a series of primary oxidation acids is formed. As is well known, S. V. Soldatenkov discovered A number of organic acids in higher plant tissues (4-, 5-, 6-, and 7-carbon acids — tetronic, heptonic, etc.). By their chemical nature and composition, they are quite close to simple sugars and represent the primary Stages of the latter's direct oxidation.

Thus, the example of glucose demonstrates that different pathways and transformation cycles exist within the cell for the oxidation of the very same compound, each catalyzed by a specific enzyme complex. Although the sugar Cleavage pathways discussed above differ significantly at their initial stages, they converge and become quite similar in subsequent reactions. It is essential to thoroughly study the relationship between various types of respiration across different plant species in relation to their ecological characteristics, developmental stages, age, tissue type, and other factors. This will provide a deeper understanding of how respiration, one of the most vital physiological functions, adapts to the living conditions of plant organisms.